Optical communication systems use optical fiber as a communication medium and light as an information carrier. For instance, an optical signal may be a beam of light modulated to represent binary-coded information. When light is used to transmit information, the information may be extracted from the beam of light through the use of a photodetector in a receiver. A photodetector is an electronic component that detects the presence of light radiation through conversion of light energy to electrical energy. A common photodetector is called a photodiode which consists of a semiconductor having a property called photoconductivity, in which the electrical conductance varies depending on the intensity of radiation striking the semiconductor material comprising the photodiode. Essentially, a photodiode is the same as an ordinary diode, except that the package has some transparency that allows light energy to effect junctions between the semiconductor materials inside.
An optical receiver using an avalanche photodiode (APD) features a built-in gain as part of the optical-to-electrical conversion process. This built-in gain makes APD receivers attractive as receivers in high bit-rate optical communication systems. However, the built-in gain requires very careful bias (operating condition) control and temperature compensation to maintain system performance over a wide range of optical input powers. This is especially true when the APD receiver does not contain any active cooling circuitry.
In U.S. Pat. No. 5,953,690, issued to Lemon et al. Sep. 14, 1999, an intelligent fiber-optic receiver is disclosed. During calibration procedures for the receiver, an optical-to-electrical conversion device (avalanche photodiode or PIN photodiode) and its supporting control and monitoring circuits in a receiver module are characterized over a defined operating temperature range. Characteristic data and/or curves defining these operational control and monitoring functions over the range of operating conditions (temperature, power supply) are stored in non-volatile memory. During operation, an embedded micro-controller, together with analog to digital converters, digital to analog converters and other associated circuitry, dynamically controls the operational constants of the module based on the current operating conditions (temperature, power supply). Unfortunately, this approach, which employs a thermal chamber while measuring receiver performance at the extremes of operating temperature, may be very time consuming and hence costly. Further, this approach may not be suited to volume manufacturing of receivers.